Underpinning a house foundation is the process of strengthening an existing foundation, usually when it has become unstable or is no longer capable of supporting the structure. This process is typically performed when there are signs of foundation settlement or shifting, such as cracks in the walls, uneven or sinking floors, or doors and windows that no longer close properly. Mainmark provides underpinning in Auckland, Wellington, Christchurch and across New Zealand.

Chemical underpinning, also referred to as resin injection, is an innovative non-invasive solution used to provide ground improvement and enhance the load-bearing capacity of a structure’s foundation.

This method is particularly useful in situations where the foundation soil has insufficient strength or when the soil’s characteristics have changed over time, reducing its ability to support a structure.

The underpinning procedure involves a series of steps designed to strengthen or stabilise the foundation of an existing building. The exact procedure can vary depending on the method of underpinning chosen (e.g., mass concrete, mini piles, etc.) and the specific needs of the building.

Underpinning is a method used to strengthen the foundation of a building, often when it has been compromised due to soil movement, settling, or other structural issues. Identifying the need for underpinning early can prevent further damage and more expensive repairs. Here are the key signs that may indicate the need for underpinning:

1. Visible Cracks in Walls or Foundation

Large or Growing Cracks: If you notice large cracks, especially those wider than 1/8 inch, that continue to grow over time, this could indicate a shifting or settling foundation. Cracks may appear in walls, ceilings, floors, or the foundation itself.

Diagonal Cracks: Cracks that appear at an angle, especially near doors, windows, or corners, may be a sign of foundation movement, requiring underpinning to stabilise the foundation.

Step Cracks: Cracks that follow the mortar lines in brick or masonry walls can signal a shifting foundation, often seen in older buildings.

2. Sinking Floors & Slabs

Sagging or Sloping Floors: If you notice that floors are uneven, sagging, or slope in certain areas, it could indicate that the foundation is settling unevenly, which may require underpinning to correct.

Bouncing Floors: Floors that feel bouncy or unstable, especially in areas near walls or corners, can be a sign that the foundation is not providing proper support.

3. Jammed Doors & Sticking Windows

Difficulty Opening or Closing Doors and Windows: If doors or windows start to stick or fail to close properly, this could indicate that the house is shifting or settling, causing misalignment of door frames or window frames.
Gaps Around Doors/Windows: If gaps develop around doors or windows (for example, at the top or bottom), it can also signal that the foundation is moving, which may require underpinning.

4. Cracking or Bowing Walls

Bowing Walls: If interior or exterior walls start bowing, bulging, or leaning, this could suggest that the foundation is not providing enough support, especially in cases of moisture-related expansion or soil instability.

Severe Wall Cracks: If the cracks are large and running horizontally across walls, this is often a serious sign of foundation instability, indicating the need for underpinning.

5. Gaps Between Walls and Ceilings or Floors

Separation Between Walls and Ceiling/Floor: If there is noticeable separation between walls and the ceiling or floor, it may indicate that the foundation is shifting or settling unevenly, which could require underpinning to stabilise the structure.

6. Sinking or Settling of the Structure

Uneven Settlement: If different parts of the house are sinking or settling at different rates, it can cause the structure to become unlevel and may be a sign of foundation issues.

Visible Foundation Movement: You may notice that the foundation itself has settled, cracked, or shifted, which can be a clear indication that underpinning is necessary to restore stability.

7. Water Damage or Drainage Issues

Water Pooling Around the Foundation: If water is pooling around the foundation or basement, it could indicate that the soil is moving or shifting, which can weaken the foundation and may lead to further settling. Proper drainage and underpinning may be needed.

Flooding or Dampness in the Basement: Excessive moisture in basements, whether from leaking pipes or poor drainage, can cause soil erosion around the foundation, resulting in structural problems that may need underpinning to address.

8. Foundation Settling Due to Soil Issues

Expansive Soils: If the foundation is built on expansive soils (clay soils that expand when wet and shrink when dry), these can cause significant foundation movement. If the soil under the foundation is unstable, underpinning may be required to prevent further damage.

Erosion or Soil Compaction: Soil erosion or inadequate compaction during construction can lead to uneven settling of the foundation. If the foundation is no longer resting on solid soil, underpinning may be necessary to provide stability.

9. Cracks in Exterior Features

Cracks in Brickwork or Masonry: Cracks in the exterior brickwork, especially vertical or diagonal cracks, can be a sign that the foundation is shifting, requiring underpinning to prevent further structural damage.

Misalignment of Exterior Elements: If exterior elements like chimneys, porches, or patios begin to lean, shift, or crack, it may indicate foundation issues that underpinning can address.

10. Soil Settlement or Foundation Inspection Reports

Professional Foundation Inspection: If a structural engineer or foundation expert has examined your home and recommended underpinning, this is a clear sign that action is required

Soil Movement: If there is evidence of soil settlement in the surrounding area (such as soil erosion, soil shrinkage, or noticeable soil displacement), underpinning may be necessary to ensure the foundation remains stable.

There are several methods of underpinning, each suited to different types of foundation problems, soil conditions, and the overall structure of the building.

The main types of underpinning are designed to strengthen a foundation, usually by extending or deepening it to more stable soil layers.

1. Mass Concrete Underpinning
2. Beam and Base Underpinning
3. Mini-Piled Underpinning
4. Screw Pile Underpinning
5. Underpinning Using Concrete Pilings
6. Resin Injection Underpinning (Chemical Injection)
7. Jet Grouting Underpinning
8. Pile and Slab Underpinning
9. Underpinning by Structural Slab Extension

The main alternatives to traditional underpinning are resin injection and screw piles.

Resin injection strengthens the soil beneath the footing by compacting the ground. Screw piles extend the footings to a deeper layer of soil.

The cost to underpin a footing can vary significantly, and depends on the method adopted, soil condition, accessibility to the failed footing and proximity of other buildings and pavements to the work area.

Traditional underpinning is labour intensive and involves large excavations, so its price does depend on how busy the contractor is. Resin Injection and screw piling have more stable prices.

Traditional underpinning required digging holes next to the footing and then filling them with concrete.

The excavation of the holes can take some time and will impact gardens and paths as they will be removed.

Traditional underpinning could take a few days for a single underpin to several weeks.

Resin injection is different to underpinning.

Resin injection involves strengthening the ground beneath the footing, whereas underpinning involves extending the footing to a deeper layer of soil.

There are several pitfalls when using concrete underpins.

In reactive clay soils: If only part of the footing is underpinned then there will be differential seasonal movement between the underpinned sections of the footing and the rest of the footings.

This difference in seasonal movement will result in large cracks forming in the walls adjacent to the areas that have been underpinned.

Another problem that arises with traditional underpinning is that poorly compacted soil adjacent to the underpin acts as a water path to the underside of the underpin, which undermines the underpin.